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What is allow snapshot isolation?
Snapshot isolation must be enabled by setting the ALLOW_SNAPSHOT_ISOLATION ON database option before it is used in transactions. This activates the mechanism for storing row versions in the temporary database (tempdb).
What is read committed snapshot?
What is Read Committed Snapshot Isolation? The idea behind Read Committed Snapshot Isolation is the following: instead of locking a record during the reading phase with a Shared Lock, SQL Server will transparently return you the old committed record version from the Version Store. The Version Store is stored in TempDb.
What’s the difference between snapshot and READ COMMITTED isolation?
To sum up, both the SNAPSHOT and the READ COMMITTED isolation levels (with READ_COMMITTED_SNAPSHOT is ON) use row versioning to manage isolation degree in transactions. While the SNAPSHOT isolation level protects from all phenomena, READ COMMITTED isolation level with enabled READ_COMMITTED_SNAPSHOT option prevents only dirty reads.
When to use the allow snapshot isolation setting?
The ALLOW_SNAPSHOT ISOLATION setting is set to ON only to allow snapshot isolation when starting a transaction (e.g. SET TRANSACTION ISOLATION LEVEL SNAPSHOT) regardless of READ_COMMITTED_SNAPSHOT setting. The only reason to have these two settings set to ON is when it needs to have READ COMMITTED row versioning AND snapshot isolation.
What does read committed snapshot do in SQL Server?
In a previous article, we discussed the READ_COMMITTED_SNAPSHOT SQL Server database option and its impact on the behavior of a transaction running under the READ COMMITTED transaction isolation level.
When READ_COMMITTED_SNAPSHOT OFF is in effect, the Database Engine uses shared locks to enforce the default isolation level. If you set the READ_COMMITTED_SNAPSHOT database option to ON, the database engine uses row versioning and snapshot isolation as the default, instead of using locks to protect the data.